What Are Salinity and Specific Gravity?
Salinity is the concentration of dissolved salts in water, usually measured in parts per thousand (ppt). Specific gravity (SG) measures the same thing indirectly, comparing the density of tank water to pure freshwater — it’s the number most hobbyist hydrometers actually display. Natural seawater sits around 35 ppt / 1.026 SG, and most reef tanks are run close to that, typically 1.023–1.026 SG.
Why Getting It Right Matters
Marine fish and especially corals and invertebrates are sensitive to salinity swings — a hydrometer that reads inaccurately (a common issue with cheap swing-arm models) can leave a tank running consistently too high or too low without the owner realizing it, causing chronic stress that’s hard to diagnose. Refractometers are generally more accurate than hydrometers and are worth the upgrade for anyone keeping corals or invertebrates.
Shop Salinity Testing
The Tropic Marin High Precision Hydrometer and the wider Marine Aquarium Test Kits range cover salinity testing, and Marine Aquarium Salt & Sand has the salt mixes to hit your target.
